1. Field of the Invention
The present invention relates to fiber lasers, and more specifically, it relates to diode-pumped alkali vapor lasers embodied as fiber lasers.
2. Description of Related Art
For applications requiring high beam quality radiation from efficient, compact and rugged sources, diffraction limited fiber lasers are ideal, and to date have been demonstrated at average power levels near 500 W with near diffraction limited output For conventional single-core, step-index single-mode fibers, this power level represents the scaling limit because of nonlinear and laser damage considerations. Higher average powers would exceed nonlinear process thresholds such as the Raman and stimulated Brillouin scattering limit, or else damage the fiber due to the high intensity level in the fiber's core. One obvious way to increase the average power capability of fibers is to increase the area of their core. Simply expanding the core dimensions of the fiber allows a straightforward power scaling due to enhanced nonlinear and power handling characteristics that scale directly with the core area. However, the enhanced power handling capability that obtains through this route comes at the expense of beam quality, as increasing the core diameter in standard step index fibers permits multiple transverse modes to lase simultaneously. Although this problem of multimode operation can be mitigated to some extent by appropriately designing the fiber's waveguide structure, limitations such as bend radius loss, sensitivity to thermally induced perturbations of the waveguide structure, and refractive index control, all become more stringent as the core diameter grows, limiting the extent to which the core diameter can be grown and still ensure single mode operation from the fiber.